Medication infusion pumps that supply a medication to a patient, in particular implantable infusion pumps, are known. Known infusion pumps typically include a housing that contains a medication chamber and a propellant chamber, together with appropriate means for loading the selected medication into the medication chamber and for supplying the medication to the patient. One known type of infusion pump includes a reservoir containing a gas at a pressure below atmospheric pressure. Such infusion pumps are characterized as "negative pressure" pumps. Negative pressure pumps offer the advantage of facilitating fill of the medication chamber by acting to draw in the medication from a hypodermic needle or other supply source into the medication chamber. Such pumps also have the advantage of inhibiting undesirable leakage from the medication chamber.
Negative pressure infusion pumps are described, for example, in U.S. Pat. No. 4,191,181, to Frenetzki et al.; U.S. Pat. No. 4,373,527, to Fischell; U.S. Pat. No. 4,511,355, to Frenetzki et al.; U.S. Pat. No. 4,573,994, to Fischell et al.; and U.S. Pat. No. 5,514,103, to Srisathapat et al. In a typical negative pressure infusion pump, a medication is drawn into the medication chamber under the action of a gas maintained in a flexible reservoir at negative pressure. Once the medication is drawn into the medication chamber, the medication is subsequently delivered to the patient via a separate pumping mechanism.
Modern environmental concerns mandate that gases employed in "negative pressure" pumps be environmentally benign. It is of particular concern to avoid gases that have an adverse effect on ozone.
A need exists for a negative pressure infusion pump that employs an environmentally acceptable gas in its gas reservoir.